Properties and roles of an intramembranous charge mobilized at high voltages in frog skeletal muscle.

1. Membrane Ca2+ currents (ICa), intramembranous charge movement currents and changes in intracellular Ca2+ concentrations were recorded in voltage clamped cut skeletal muscle fibres of the frog. Intra- and extracellular solutions, designed to prevent ionic current, and use of the saponin-permeabilization procedure made possible the measurement of transfer of intramembranous charge up to high positive potentials. 2. Substantial charge moved at positive potentials. This charge was shown to be intramembranous in four tests of charge conservation, demonstrating that the total displacement of charge depended only on the initial and final voltages, and not on the history or pathway of intermediate voltages. 3. On average, in twenty-three cells, the charge moved at 50 mV was 31 +/- 1.9 nC microF-1 (mean +/- S.E.M.), and at 0 mV was 25 +/- 1.5 nC microF-1. Approximately one-fifth of the total charge moved above 0 mV. 4. The charge that moved at high voltage could be fitted, in most cases, with a Boltzmann distribution function. In twenty of twenty-three cells, the total charge distribution could be fitted as the sum of two Boltzmann terms; the high voltage term was centred at 11 +/- 3.9 mV, with a steepness factor of 12 +/- 1.6 mV and a magnitude of 8.6 +/- 1.1 nC microF-1. The low voltage term was centered at -43 +/- 2.1 mV, with a steepness factor of 7.7 +/- 0.6 mV and a magnitude of 22 +/- 1.8 nC microF-1. Thus, the high voltage component comprised about one-quarter of the mobile charge. In four cells it was possible to fit the sum of three Boltzmann terms to the distribution of mobile charge; the parameters of the high voltage term then were similar to those found by fitting the sum of two Boltzmann terms to the same data. 5. The voltage dependence of activation of ICa was determined in a buffered 2 mM Ca2+ external solution, from the tails of ionic current at -30 mV, after activating pulses to various voltages, the duration of which was sufficient to reach the peak of inward current. The voltage dependence was described by a Boltzmann function centred at 2.6 +/- 6.9 mV (n = 6), with a steepness factor of 20 +/- 1.4 mV. The voltages at which the high voltage charge moved were roughly the same as those at which ICa was activated. 6. Calcium release from the sarcoplasmic reticulum was determined from the Ca2+ transients. Calcium release continued to increase at potentials above 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)